In the raster scan display art it is well known to keep the vertical and horizontal sweep oscillators of a display in phase synchronization with signal generating components of a video signal generator. In the United States television equipment and raster scan video display terminals function in synchronization with the 60 hertz commercial power to eliminate the problem known in the art as "line frequency beating" evidenced by a vertical waviness in a video display. Typically, a horizontal sweep oscillator of television receivers and displays oscillate at 15.75 kilohertz and the vertical sweep oscillator oscillates at 60 hertz. In television transmitting stations crystal oscillators are generally used to generate the vertical and horizontal synchronization signals that are used to conrtol the horizontal and vertical deflection signal generators. Synchronization of a display or receiver with video signal synchronization signals is accomplished by frequency comparator circuits which are utilized to compare the frequency of local vertical and horizontal oscillators circuits with the synchronization pulses of the video signal input to the display or receiver. The most widely used frequency comparator circuit is a phase lock loop circuit.
In the prior art, the vertical and horizontal oscillators of raster scan displays have been designed to operate at a given frequency synchronous with harmonics of the public utility power line to prevent line frequency beating and cannot operate other power line frequencies. In Great Britain and other countries the power line frequency is 50 hertz (hz), while in the United States the power line frequency is 60 hz. In the prior art displays designed to operate synchronously with both power line frequencies had to have their sweep oscillator circuit configurations manually or automatically changed to change the frequency of oscillation thereof. U.S. Pat. No. 3,129,390 issued Apr. 14, 1964 to R. C. March et al discloses a switch used to change the circuit configuration of the sweep oscillators of the raster scan display.
It is an object of this invention to provide a new and improved raster scan display that does not require a frequency comparator circuit for locking the frequency of the vertical and horizontal sweep oscillators to the synchronization pulses of a video signal.
It is another object of this invention to provide a new and improved raster scan display that can operate synchronously with different public utility power line frequencies without changing the frequency of operation of the sweep oscillators.
It is a further object of this invention to provide a new and improved raster scan display that will not "beat" against the public utility power line resulting in a distorted display.